The present invention relates to a gas laser apparatus of the type wherein a pair of discharge electrodes are disposed so as to be placed in an atmosphere of a gas mixture as a laser medium, and more particularly to an improvement of a material of the discharge electrodes of the gas laser apparatus.
As the gas laser of the above-mentioned type, a CO.sub.2 laser has been used for material processing. The CO.sub.2 laser generally comprises a discharge section filled with a gas mixture as a laser medium, a plurality of anode bars provided in the discharge section so as to be arranged in the shape of a reed screen, and a plurality of generally L-shaped cathode bars disposed in opposite relation to the anode bars. A high voltage DC power supply is connected between the discharge electrodes through ballast resistances. DC voltage is applied across the discharge electrodes, causing a glow discharge to take place between the electrodes and thereby exciting the gas mixture. The gas mixture is recirculated through a heat exchanger from the cathode side to the anode side for the purpose of preventing the temperature thereof from being increased. Molybdenum has been conventionally employed as a material of both of the discharge electrodes because of its high melting point.
In the laser apparatus introduced to a production line of a works for the purpose of the material processing not studying, it is desirable that the maintenance cycle of the laser apparatus be as long as possible since the maintenance cycle affects productivity of the production line. While, in the conventional CO.sub.2 laser, the discharge electrodes are deteriorated and the electric discharge in the discharge section of the laser is altered from the glow to arc discharge when the maintenance of the electrodes is not executed. Under these circumstances, the maintenance of the discharge electrodes is reiteratively executed in a relatively short cycle. For example, the conventional transverse flow 5-kW gas laser is continuously used for about 200 hours at the maximum.
The inventors of the present invention made experiments to investigate the mechanism of deteriorating the discharge electrodes. From the experiments, the inventors have found that a physical sputtering action is applied to the cathode formed from molybdenum owing to collision of positive ions during the glow discharge. Further, since molybdenum has a sublimation temperature as low as below 700.degree. C., molybdenum is also likely to suffer from the chemical sputtering action. Consequently, the discharge electrodes are deteriorated owing to both physical and chemical sputtering actions. Inspection of the molybdenum cathode after a long time of use shows that net-shaped projections are formed along grain boundaries on the surface of the molybdenum cathode, which makes it clear that the sputtering acts on the cathode ununiformly. Further, a thin oxidized film is formed on the surface of the cathode. It is considered that the thin oxidized film results from an extremely small amount of oxygen contained in the gas mixture in which He, N.sub.2, and CO.sub.2 are in the ratios of 50:45:5. It is further considered that the oxygen results from leakage through sealing members of an airtight vessel of the gas laser apparatus or decomposition of CO.sub.2 contained in the gas mixture.
On the other hand, inspection of the molybdenum anode after a long time of use shows that a piled or stacked substance about 1 or 2 .mu.m thick is found on the side thereof confronting the cathode and that black projections, the outer diameter of each of which is about 100 .mu.m long are found on the surface thereof. Since the piled substance is amorphous MoO.sub.3 and partial to the side of the anode confronting the cathode, it is considered that the above-described sputtering in the cathode causes cathode material particles to scatter and move to the anode with a flow of the gas mixture, resulting in the substance piled up on the anode. Analysis shows that projections formed on the piled substance is composed of carbon. Consequently, it is considered that micro-arcs are generated from the projections.
From the above-described results of analysis, mechanism leading from the deterioration of the discharge electrodes to the generation of micro-arcs is considered as follows: the molybdenum cathode suffers from the sputtering action owing to the glow discharge in the discharge section of the gas laser, resulting in the substance ununiformly piled up on the anode. Since the piled substance is a molybdenum oxide, it has an insulating property. Concentration of the electric field takes place on some portions of the anode surface. While, the net-shaped projections are formed on the surface of the cathode owing to the ununiform sputtering action, which projections also cause concentration of the electric field. The micro-arcs are generated as the result of the electric field concentration and the electric field tends to be concentrated on the micro-arc generating points as the number of the micro-arc generating points is increased, whereby ununiformity of the discharge input progresses. The discharge input locally reaches the arc limit or above and at last, the glow discharge is altered to the arc discharge throughout the discharge section. The above-described condition is applied not only to the gas lasers of the DC discharge type but to those of the AC discharge type, wherein both of the discharge electrodes alternately suffer from the sputtering to thereby be worn and deteriorated.
So far as described, the conventional gas laser has a problem that the oxide produced owing to the sputtering at one discharge electrode side is piled up on the surface of the other discharge electrode and that deterioration of the electrode lowers the arc limit, thereby requiring the maintenance in a short period.